Aviation safety investigations & reports

Saab Aircraft AB SF-340B, VH-OLL

Summary

At 1353 Eastern Standard Time, a Saab 340 departed Sydney for
Tamworth climbing to flight level (FL) 140. The flight was
conducted within controlled airspace and crossed the boundary
between the Sydney and Brisbane Flight Information Regions (FIR),
45 nm north of Sydney. The airline had chosen to use the aircraft
registration instead of the flight number as the callsign. This
flight stage was one of six separate flight plans held within The
Australian Advanced Air Traffic System (TAAATS) for the aircraft.
The flight plans had varying departure times, two of which were for
Sydney-Tamworth flights. Companies that chose to use flight numbers
as the aircraft callsign would have had one unique flight data
record for each flight stage held within TAAATS.

TAAATS used a messaging system for flights that cross the FIR
boundary, this system used messages called ASIA/PACIFIC ATS
Inter-Facility Data Communications (AIDC). The normal AIDC system
messaging occurred between the Melbourne and Brisbane Flight Data
Processors (FDP). However, as there were several plans in the
system for the Saab 340, the Brisbane FDP did not send an accept
message (ACP) to the Melbourne FDP. In the absence of an ACP from
Brisbane, the Melbourne FDP created an unsuccessful coordination
"U" warning in the Sydney controller's label and an aural alarm was
generated, indicating that coordination message exchange between
Brisbane and Melbourne FDP's was unsuccessful.

The unsuccessful (undeliverable) AIDC coordination messages
between the two FDP's were automatically sent to the BN Flight Data
Coordinator (TFDC). The Manual of Air Traffic Services (MATS)
required that the TFDC print these messages and give them to the
controller concerned, however on this occasion the TFDC did not
pass on these messages.

The Sydney controller received and acknowledged the "U" warning
and alarm. Once the alarm had been acknowledged, the system removed
the "U" warning from the screen and the controller then had no
visual indication that the aircraft had been the subject of an
unsuccessful coordination alarm.

MATS required that aircraft that are subject to a "U" alarm are
to be verbally coordinated to the sector in the other FIR. The
voice coordination for the Saab 340 did not occur. The alarm was
also not mentioned by the Sydney controller during a hand-over to
another Sydney controller.

The second Sydney controller initiated the electronic radar
hand-off on the Saab 340 to the Brisbane controller. Without
waiting for formal acceptance from the Brisbane controller, the
Sydney controller instructed the crew to contact Brisbane Air
Traffic Control. This was not in accordance with procedures.

The crew of the Saab 340 attempted to contact the Brisbane
Maitland (MND) sector controller three times. During the first
transmission, the controller was conducting coordination with
another unit, and did not hear the aircraft. The second time the
crew attempted to contact MND, another pilot over transmitted and
the controller did not hear the transmission. The third time, the
crew only used the callsign of the aircraft and did not advise
their flight level. The controller instructed the crew to
"standby".

The MND controller was under training, and had only just
returned to controlling duties after a long absence. The MND
controller did not call the crew of the Saab 340 back.

As the Brisbane FDP had not processed the flight data record
(FDR) for the Saab 340, the aircraft appeared on the MND air
situation display (ASD) as a black "unconcerned" track. This track
would only have displayed a secondary surveillance radar code and
level in the label. It did not display the aircraft's call
sign.

When the Saab 340 entered Brisbane's airspace 45 nm north of
Sydney, there were numerous other black tracks outside controlled
airspace (OCTA) on the MND sector ASD.

At the time the crew of the Saab 340 were attempting to contact
the MND controller, the aircraft's label was superimposed on
another black track directly underneath it. It is a common
occurrence to have aircraft labels overlying each other. TAAATS
provides functionality to enable the controllers to move the
aircraft labels to reduce label clutter.

The aircraft then passed through Nambucca (NAA) and Armidale
(ARM) Sectors, as a black track in controlled airspace, without
being detected by the controllers.

Recorded radar data indicated that the Saab 340 did not pass
adjacent to any other aircraft and consequently there was no
infringement of separation standards. The first time that the
Armidale (ARL) sector controller became aware of the aircraft was
when the crew of the Saab 340 eventually contacted the MND
controller for descent into Tamworth. The aircraft was subsequently
radar identified and processed normally into Tamworth.

Safety Action

LOCAL SAFETY ACTION

As a result of this occurrence, Airservices Australia has modified the software in TAAATS to improve the way that a flight data record can be selected from the flight plan window where multiple flight data records exist and has improved the AIDC processing by changing the logic used in uniqueness checking. These software modifications were approved for release for operational use in September 2001.